Internal combustion engines of a vehicle may operate in various combustion modes. One example mode is spark ignition (SI), where a spark performed by a sparking device is used to initiate combustion of an air and fuel mixture. Another example mode is homogeneous charge compression ignition (HCCI), where an air and fuel mixture achieves a temperature where autoignition occurs without requiring a spark from a sparking device. In some conditions, HCCI may have greater fuel efficiency and reduced NOx production compared to SI. However, in some conditions, such as with high or low engine loads, it may be difficult to achieve reliable HCCI combustion.
One approach in this field is US Pub. No. 2005/0173169, which uses a dual combustion mode engine configured in a hybrid vehicle propulsion system, wherein the engine is configured to operate in HCCI mode under some conditions and SI mode under some conditions, while the hybrid system may be used in conjunction with the engine to reduce transitions between combustion modes and provide the requested output to the vehicle drive wheels.
However, the inventors herein have recognized a disadvantage with such an approach. In some conditions, such as when a lean NOx trap requires purging, the HCCI mode, even if complemented by the hybrid system, may not provide appropriate exhaust conditions to facilitate the purging process. Thus, the lean NOx trap may not be correctly purged thereby degrading the performance of the trap.
Thus, it may be advantageous to adjust combustion modes in a hybrid vehicle to provide improved operation of a lean NOx trap. In one approach, such issues may be addressed by a hybrid vehicle propulsion system, comprising an engine having at least one combustion cylinder configured to operate in a first and a second combustion mode, wherein the first combustion mode is a spark ignition mode and the second combustion mode is a compression ignition mode, wherein the engine is configured to supply an engine output; a motor configured to selectively supply or absorb a secondary output; an energy storage device configured to selectively store at least a portion of the engine output and selectively supply energy to the vehicle; a lean NOx trap configured to store NOx produced by the engine; and a controller configured to control at least the engine, the motor, and the energy storage device, wherein the controller is configured to selectively operate said at least one combustion cylinder in one of the spark ignition mode and the compression ignition mode based on a condition of the lean NOx trap.
In this way, it is possible to reduce the frequency of some transitions between combustion modes by operating the hybrid propulsion system to supply or absorb output as needed while periodically performing a transition to spark ignition to facilitate purging of the lean NOx trap. Thus, the overall transitions between modes, and selection of modes, may be managed to take into account interaction of the hybrid drive system and emission control device requirements.